It is well known that silver as a precious metal has desirable electrical and thermal conductivity, catalytic properties, and antimicrobial behavior. Thus, silver and silver-containing compounds have been widely used in alloys, metal plating processes, electronic devices, imaging sciences, medicine, clothing or other fibrous materials, and other commercial and industrial articles and processes to take advantage of silver's beneficial properties.
For example, silver compounds or silver metal have been described for use as metallic patterns or electrodes in metal wiring patterns, printed circuit boards (PCBs), flexible printed circuit boards (FPCs), antennas for radio frequency identification (RFID) tags, plasma display panels (PDPs), liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), flexible displays and organic thin film transistors (OTFTs), among other electronic devices known in the art.
Rapid advances are also occurring for making and using various electronic devices for various communication, financial, and archival purposes.
Silver is an ideal conductor having electrical conductivity 50 to 100 times greater than indium tin oxide that is commonly used today in many devices. For example, the art has described the preparation of electrically-conductive films by forming and developing (reducing) a silver halide image in “photographic” silver halide emulsions through an appropriate mask to form electrically-conductive grid networks having silver wires having average sizes (width and height) of less than 10 μm and having appropriate lengths. Various efforts have been made to design the silver halide emulsions and processing conditions to optimize electrically-conductive grid designs.
While silver as an electrical conductor has a wide range of potential uses in the field of printed electronics, the microfabrication of electrically-conductive tracks (grids, wires, or patterns) by photolithographic and electroless techniques is time consuming and expensive, and there is an industrial need for direct digital printing to simplify the processes and to reduce manufacturing costs.
Furthermore, it is desirable to fabricate silver-containing electronics onto polymeric or similar temperature-sensitive substrates by solution-based printing processes. Metallic electrically-conductive wires or grids of low resistance must be achieved at sufficiently low temperatures to be compatible with organic electronics on polymeric substrates. Among various known methods for fabricating electrically-conductive silver grids or patterns, the direct printing of silver-containing inks provides attractive prospects for making such electrically-conductive patterns.
Commonly used silver-conductive inks useful for this purpose are currently based or dependent upon the presence of silver nanoparticle (NP) solutions or dispersions, all of which have associated drawbacks. To overcome the common problem of aggregation and flocculation in silver nanoparticle based inks, various thiolate encapsulating surfactants or dispersants can be used. Volkman et al. [Chem. Mater. 23, 4634-4640 (2011)] observed that a thiolate encapsulating surfactant could be used to treat 3 nm silver particles in silver-containing inks to achieve films sintered at temperatures above 175° C. in air. Sintering is essential to obtain the electrical conductivities required for electronic applications. The effects of sintering on electrical performance and microstructure for an inkjet-printed copper nanoparticle ink were explored by Niittynen et al. [Sci. Rep. 5, 8832 (2015)]. These workers used laser and intense pulsed light (IPL) sintering to obtain articles having electrical conductivities greater than 20% of that of bulk copper.
However, sintering techniques have major disadvantages. In many cases, sintering steps require high temperatures that are not compatible with polymer substrates such as polyethylene terephthalate or polycarbonate that are commonly employed in many consumer electronic articles. Furthermore, the metal-containing inks used for these processes have disparate viscosities and synthetic parameters. Particle-based inks typically contain electrically-conductive metal particles that are synthesized separately and then incorporated into an ink formulation. Each resulting particle-based ink must then be optimized for use in a specific printing process.
Grouchko et al. [ACS Nano 5(4) 3354-3359 (2011)] recently overcame some of these problems by employing a room temperature, “built in” sintering mechanism that successfully produced silver metal articles exhibiting electrical conductivities as high as 41% of the electrical conductivity of bulk silver. To obtain these electrical conductivity values, a chloride salt (such as NaCl) or HCl vapor was employed to strip a polymeric (polyacrylic acid sodium salt) electrosterically stabilizing coating from the ˜15 nm diameter silver nanoparticle feedstock. This sintering mechanism consisted of spontaneous coalescence and Ostwald ripening, driven by the surface-to-volume energy of the very small silver nanoparticles. Thus, these nanoparticle-based processes inherently involve sintering processes, whether they are chemical (for example using a strong acid such as hydrochloric acid), thermal, laser, or UV activated.
Inkjet printing and flexographic printing have also been proposed for providing patterns of silver or silver-containing compounds, requiring the careful fabrication of a silver-containing paste or “ink” with desirable surface tension, viscosity, stability, and other physical properties required for such application processes. High silver content has generally been required for high electrical conductivity, and calcination or sintering may be additionally required for increasing electrical conductivity of printed silver inks.
An alternative to the approaches described above is to employ a chemical ink formulation where the silver source is a molecular precursor or cation (such as a silver salt) that is then chemically reacted (or reduced) to produce silver metal. Electrically-conductive inks that are in the form of a chemical solution rather than as a suspension or dispersion of metal particles, have gained interest in recent years [see for example Walker and Lewis in J. Am. Chem. Soc. 134, 1419 (2012); and Jahn et al. Chem. Mater. 22, 3067-3071 (2010)]. One conductive ink of this type is known as a Metalorganic Decomposition (MOD) variety ink, for example, as described by Jahn et al. [Chem. Mater. 22, 3067-3071 (2010)] who investigated silver printing using an aqueous transition metal complex [AgO2C(CH2OCH2)3H]-containing MOD ink. They reported the formation of metallic silver features having electrical conductivities as high as 2.7×107 S m−1, which corresponds to an electrical conductivity that is 43% of that of bulk silver, although a sintering temperature of 250° C. was required. MOD inks thus overcome some problems associated with the use of nanoparticle-containing inks, for example, nozzle clogging, but numerous printing passes are generally required to obtain an adequate sheet resistance. Post-treatment sintering processes are also still required to fully consolidate the electrically-conductive articles if the growth process is initiated from discrete nanoparticle intermediates, which is common in MOD ink processes.
U.S. Patent Application Publication 2015/0004325 (Walker et al.) describes a chemically-reactive silver ink composition comprised of a complex of a silver carboxylate salt and an alkylamine, in which the complex is used to form an electrically-conductive silver structure at a temperature of 120° C. or less. Unfortunately, even these temperatures render the ink incompatible with many polymeric and paper substrates used in flexible electronic and biomedical devices. Furthermore, since alkylamines are known to reduce silver at room temperature, long term stability of such compositions is tentative. The complexes must be kept in air-tight refrigerated storage for extended keeping stability (Column I, paragraph 0054 of the publication). Furthermore, the publication teaches long heating times were needed to obtain low resistivity in the resulting articles.
A common coordinating ion to form organic silver complexes is carboxylic acid [Prog. Inorg. Chem., 10, 233 (1968)]. However, silver-carboxylate complexes are generally insoluble in organic solvents [see, for example, U.S. Pat. No. 5,491,059 of Whitcomb and U.S. Pat. No. 5,534,312 of Hill et al.] and have a high decomposition temperature. To solve this problem, several methods have been proposed for example, in Ang. Chem., Int. Ed Engl., 31, p. 770 (1992), Chem. Vapor Deposition, 7, 111 (2001), Chem. Mater., 16, 2021 (2004), and U.S. Pat. No. 5,705,661 (Iwakura et al.). Among such methods are those using silver carboxylates having long alkyl chains or including amine compounds or phosphine compounds. However, the silver complexes known thus far have insufficient stability or solubility and a high decomposition temperature is needed for pattern formation and are decomposed slowly.
Allegedly to address some of these problems, U.S. Pat. No. 8,226,755 (Chung et al.) describes silver complexes formed by reacting a silver compound (such as a silver salt) with an ammonium carbamate compound or ammonium carbonate compound. Moreover, U.S. Patent Application Publication 2010/0021704 (Yoon et al.) describes the preparation and use of fatty acid silver salts complexed with amines and in admixture with silver oxide to form silver metal from the silver oxide at low temperature.
U.S. Pat. No. 8,163,073 (Chan et al.) describes the use of silver ammonium complex ions, silver amine complex ions, silver-amino acid complex ions, silver halide complex ions, silver sulfite complex ions, or silver thiosulfate complex ions for silver plating processes to form silver wires for various devices.
U.S. Pat. No. 7,682,774 (Kim et al.) describes other photosensitive compositions comprising silver fluoride-organic complex precursors as catalyst precursors as well as the use of polymer derived from a monomer having a carboxyl group and a co-polymerizable monomer that may provide polymeric stability and developability of the resulting “seed” silver catalyst particles used for electroless plating.
U.S. Pat. No. 8,419,822 (Li) describes a process for producing carboxylic acid-stabilized silver nanoparticles by heating a mixture of a silver salt, a carboxylic acid, and a tertiary amine. However, it has been observed that such silver-containing complexes are not thermally or light stable. The reducible silver ions are readily reduced under ambient light conditions, and the resulting electrical conductivity of silver particles is minimal.
Other industrial approaches to preparing electrically-conductive films or elements have been directed to formulating and applying photocurable compositions containing dispersions of metal particles such as silver metal particles to substrates, followed by curing of the photocurable components in the photocurable compositions. The applied silver particles in the cured compositions thus act as catalytic (seed) particles for electrolessly plated electrically-conductive metals. Useful electrically-conductive grids prepared in this manner are described for example, in U.S. Pat. No. 9,188,861 (Shukla et al.) and U.S. Pat. No. 9,207,533 (Shukla et al.) and in US Patent Application Publications 2014/0071356 (Petcavich) and 2015/0125596 (Ramakrishnan et al.). Using these methods, photocurable compositions containing catalytic silver particles can be printed and cured on a suitable transparent substrate, for example, a continuous roll of a transparent polyester, and then electroless plating can be carried out on the catalytic silver particles. However, these methods require that high quantities of silver particles be dispersed within the photocurable compositions in a uniform manner so that coatings or printed patterns have a sufficiently high concentration of catalytic sites. Without effective dispersing, silver particles readily agglomerate, leading to less effective and uniform application of catalytic metal patterns and electroless plating.
Despite the various approaches and efforts to provide electrically-conductive silver in various consumer and industrial articles described above, there remains a need for thermally sensitive silver-generating compositions and processes which can rapidly generate metallic silver under appropriate heating conditions. Ideally, such thermally sensitive compositions should have several properties: stability at room temperature for an extended time (that is, limited self-reduction of silver ions); capability of being deposited using a wide range of application processes, whether uniformly or patternwise; useful at room temperature; and controllable chemical activity.